# Two Pointers

> Solve pair-sum, sliding window, and cycle detection problems with two pointers. Use when reducing O(n²) nested loops to O(n) on sorted or sequential data.

- Skill: `knoopx/two-pointers` (Agent Skill)
- Install (CLI): `npx skillmds@latest add knoopx/two-pointers`
- Raw SKILL.md: https://api.skillmd.com/api/skills/knoopx/two-pointers/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: AI & ML
- Author: knoopx (https://skillmd.com/u/knoopx)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/knoopx/two-pointers

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## When to use

Two pointers on a sorted array: start left=0, right=n-1, move inward based on comparison.

## Rules

- Solves pair-sum, three-sum, container problems in O(n)
- Sliding window: expand right boundary, shrink left when constraint violated
- Solves "longest/shortest substring with property" in O(n)
- Fast/slow pointers: detect cycles in linked lists (Floyd's), find middle element
- If brute force is O(n^2) nested loops over a sorted or sequential structure, two pointers likely reduces it to O(n)
- ALWAYS prefer two pointers over nested loops when data is sorted
- NEVER use nested loops when two pointers would work

## Complexity

Two pointers: O(n). Sliding window: O(n). Fast/slow: O(n).

## Example

"Two sum on sorted" → `left=0, right=n-1`; if `arr[left]+arr[right] < target`: left++, else right--. "Longest substring without repeats" → expand right, shrink left when duplicate found.